There has been renewed interest in the genetics of speciation, yielding many theoretical and empirical results. However, most mapping studies of polygenes responsible for morphological/behavioral differences between species have had very crude resolution, only down to the level of very large chromosome segments. The actual genes responsible for these evolutionary changes are unknown. A trait involved in Drosophila mating choice, male sex comb, is an ideal model system for further progress. Sexual selection has generated differences between sibling species that exhibit only partial postzygotic isolation, enabling genetic analysis. Moreover, the biochemical and developmental pathways responsible for sex comb formation are among the best understood, with a large choice of candidate genes. Our long-term goal is to dissect the molecular genetic basis of interspecific differences using sex comb evolution as a model. We will capitalize on the region 73A-84BD, known to contribute half of the tooth number difference between D. simulans and D. mauritiana. This region also affects bristle number on the anal plate, clasper, and the 5th sternite. First, we will increase the mapping precision ten fold to determine whether these effects are due to on e major gene with pleiotropic effects, several genes with trait-specific effects, or several genes with smaller, additive or epistatic, effects. Second, we will identify these quantitative trait genes (QTG) using P element hopping and mapping based on site-specific P element- induced male recombination. Third, we will compare the alleles contributing to intraspecific variation with those causing interspecific differences. We will sample 100 natural D. simulans QTG alleles and substitute them into the otherwise identical genetic background within 100-200kbp chromosome intervals. This will determine the distribution of effects of natural QTG alleles. We will identify molecular polymorphisms segregating between QTG alleles that modify sex comb, and find their frequencies in both D. simulans and D. mauritiana. Finally, we will test whether these QTG alleles contribute to pre- and postmating reproductive isolation between species. This comprehensive assessment of the genetics of several traits will further our understanding of the genetics of speciation.